As is known in the art, a basic component of an MTJ device is a sandwich of two thin ferromagnetic (and/or ferrimagnetic) layers (magnetic free layer and a magnetic pinned layers) separated by a very thin insulating layer (tunnel barrier layer) through which electrons can tunnel. The tunneling current is typically higher when the magnetic moments of the magnetic free and pinned layers are parallel (oriented in same direction) and lower when the magnetic moments of the magnetic free and pinned layers are anti-parallel (oriented in opposite directions). The change in conductance for these two magnetic states can be described as a magnetoresistance. In general, the tunneling magnetoresistance (TMR) of an MTJ is defined as (RAP-RP)/RP where RP and RAP are the resistance of the MTJ for parallel and anti-parallel alignment of the magnetic free and pinned layers, respectively.
In general, spin torque MRAM (Magnetoresistive random access memory) is a 2-terminal device that utilizes a magnetic tunnel junction stack to provide a non-volatile memory. An MRAM memory cell comprises a magnetoresistive structure that stores a magnetic moment that is switched between two directions corresponding to two data states (“1” and “0”). In an MRAM cell, information is stored in magnetization directions of a free magnetic layer with the magnetization of the pinned layer maintained in a fixed direction. In a conventional spin-transfer MRAM memory cell, the data state is programmed to a “1” or to a “0” by forcing a write current directly through the stack of layers of materials that make up the MRAM cell.
For example, with the magnetization of the magnetic pinned layer in a fixed direction, a current passed in one direction through the magnetic tunnel junction makes the magnetic free layer parallel to the magnetic pinned layer (low resistance state), while a current passed through the magnetic tunnel junction in the opposite direction makes the magnetic free layer anti-parallel to the magnetic pinned layer pinned layer (high resistance state). A smaller current (of either polarity) is used to read the resistance of the device, which depends on the relative orientations of the free and pinned layers.
Spin torque MRAM technology faces several challenges before it can become practical for manufacture and implementation of MRAM memory devices A key issue is the need to reduce the write current needed to switch the MTJ structure of the MRAM device between low and high resistive states (between logic low and high levels). A simple single domain model as discussed below provides guidance on optimum material parameters for spin torque MRAM. For example, a formula for the switching voltage has been derived as follows:
            V      CO        =                            2          ⁢                                          ⁢          q                ℏ            ⁢                                    RA            ⁢                                                  ⁢            α            ⁢                                                  ⁢                          M              s                        ⁢            t                                η            sp                          ⁡                  [                                    H              k                        +                          2              ⁢              π              ⁢                                                          ⁢                              M                s                                              ]                                E      a        =                  1        2            ⁢              M        s            ⁢              tAH        k            (see J. Z. Sun et, al, “Spin Angular Momentum Transfer in a Current-Perpendicular Spin-Valve Nanomagnet,” Proc. SPIE Vol. 5359, pp. 445-455, in Quantum Sensing and Nanophotonic Devices, edited by Manijeh Razweghi, Gail J. Brown (SPIE, Bellingham, Wash., 2004)).
In the above equations, VCO denotes a switching voltage threshold and wherein Ea denotes an activation energy. In the above equations, the parameter q denotes electron charge, h is planck's constant, RA denotes a resistance-area product of the tunnel barrier, α is a magnetic damping factor, Ms denotes a magnetization of the magnetic free layer, t denotes a thickness of the magnetic free layer, ηsp denotes the spin polarization, Hk denotes an in-plane uniaxial anisotropy, and 2πMs is the half the easy-plane anisotropy.
In the above equations, it is desirable to reduce the value of VCO while maintain the activation energy Ea large enough to prevent spontaneous fluctuations between the different states due to thermal fluctuations and provide a long memory lifetime, e.g., 10 years. In the equation for VCO, the value of
      2    ⁢                  ⁢    q    ℏis a constant that does not change. The value of RA depends on the thickness of the tunnel barrier. However, the tunnel barrier cannot be made too thin because of potential damage or breakdown (hole formation) of the material layer. The value of ηsp, the spin polarization, is a value that ranges from 0-1, and cannot be larger than 1. The value of the magnetic damping, α, can be controlled but cannot be made too small, as the magnetic device can oscillate/ring. In this regard, it is not feasibly practical to reduce the values of
      2    ⁢                  ⁢    q    ℏor RA or α to reduce VCO.
The values of the activation energy Ea and VCO are both directly proportional to the values of t (thickness of the magnetic free layer) and Hk (in-plane uniaxial anisotropy). One way to decrease VCO is to reduce the thickness, t, of the magnetic free layer. However, a reduction in the thickness, t, of the magnetic free layer, will result in a reduced activation energy Ea, which is undesirable. However, by simultaneously decreasing t and increasing the value of Hk, the value of the activation energy Ea can be maintained high, while reducing the value of VCO.
Indeed, although the value of VCO depends on the value of Hk, the value of Hk has greater effect on the value of the activation energy Ea than VCO. This is because the value [Hk+2πMs] determines the switching voltage, and Hk typically is of the order 100 Oe and 2πMs is of order 10,000 Oe, so that the switching voltage is dominated by the 2πMs term. So increasing the value of Hk results in a greater increase of Ea than VCO, relatively speaking. In this regard, one method to reduce VCO while maintaining the activation energy Ea relatively constant is to simultaneously reduce t and increase Hk.